A Wal-Mart spokeswoman said the company holds suppliers to high standards… (Kevork Djansezian, Getty…)

Thirty workers didn't report for their jobs Wednesday at a warehouse supplying Wal-Mart Stores Inc. and other retailers, protesting alleged retaliation from managers after employees complained about poor working conditions, organizers said.

The action in Mira Loma is the latest in a string of protests against warehouses and logistics companies in the Inland Empire by Warehouse Workers United, a union-backed advocacy group pushing for better working conditions.

The workers, who aren't represented by a union, staged a rally at a warehouse operated by Olivet International Inc., which sells apparel and luggage to various retailers, including Wal-Mart, Target Corp. and Sears.

The state's Division of Occupational Safety and Health, or Cal/OSHA, said it is investigating a complaint submitted in May by workers who said working conditions were inadequate, spokesman Peter Melton confirmed Wednesday. The state has six months to complete its investigation, he said.

Workers have complained about inadequate access to drinking water, lack of ventilation and blocked fire exits, among other conditions, said Elizabeth Brennan, a spokeswoman for Warehouse Workers United.

Olivet Corporate Secretary Peter Lin said that the company is committed to a safe workplace and that allegations of retaliation are false.

"The working conditions are nothing like those claimed," Lin said. "We are going through an OSHA inspection and have been cooperating fully with OSHA. Any issues that arise as a result of OSHA's inspection will be addressed accordingly."

Lin noted that most of Olivet's 230 employees at the warehouse opted not to participate in the work action.

The Warehouse Workers United advocacy group has singled out Wal-Mart, saying it is one of Olivet's biggest clients and that it has the weight to change industry practices.

A Wal-Mart spokeswoman said the company holds suppliers to high standards and that it takes allegations of poor working conditions seriously, even if the workers who walked off the job Wednesday aren't direct employees of the retail giant. Wal-Mart is conducting its own investigation, she said.

Still, "today is nothing more than a publicity stunt conjured up by the unions attempting to gain attention," said Brooke Buchanan, a spokeswoman for Wal-Mart.

The Inland Empire is home to dozens of mega-warehouses, particularly near Ontario Airport in San Bernardino County. The logistics industry, which includes warehousing, trucking and wholesale trade, is a major employer in San Bernardino and Riverside counties.

The industry accounts for about 114,000 jobs in the region, according to economist John Husing.

The warehouses receive containers of manufactured products from the ports of Los Angeles and Long Beach and re-load the goods onto trucks bound for big-box outlets, such as Wal-Mart and Target.

Many of the warehouses are operated by independent contractors. Wal-Mart, for instance, owns a warehouse complex in Mira Loma but contracts with a unit of Schneider National Inc. to run it. Schneider, in turn, has contracts with Impact Logistics Inc. to unload the shipping containers and Premier Warehousing Ventures to load the outbound trucks.

In late 2011, the state fined labor subcontractors Impact Logistics and Premier Warehousing Ventures more than $1 million for failing to maintain proper pay records and itemized pay statements for hundreds of temporary workers. Neither Schneider nor Wal-Mart was cited.

Workers also filed a class-action lawsuit that year against those contractors, alleging wage theft. They eventually added Wal-Mart as one of the defendants. The retailer tried to block that action, but a federal judge in January ruled Wal-Mart can be named in that class-action suit.

A report released last summer by the National Employment Law Project, which advocates for low-wage workers, found that Wal-Mart has significantly outsourced its supply chain, hiring third-party companies to operate its warehouses and transport its goods to stores. That has led to reduced wages for workers in the industry.

The workers protesting conditions at Olivet International are also demanding better wages.

Cesar Garcia, 27, was among the workers who walked off the job Wednesday. He said he has worked at the warehouse for six years.

During the summer months, the warehouse temperature gets so hot because of poor ventilation that some workers have fainted, Garcia said. The work is grueling, and he is paid $9.50 per hour, working three days a week.

"We're not asking for conditions that are out of this world," he said in a telephone interview after the rally. "We're just looking to be treated with respect and dignity and to have safe working conditions."

Garcia said management began cracking down on workers after they reported the poor working conditions, installing more than two dozen cameras around the property to monitor employees. Managers have also issued new rules aimed at keeping workers from discussing health and safety problems

The most egregious problems, Brennan said, have been fixed. Fire exits are no longer blocked, she said.

The strike comes about a year after workers at another supplier warehouse in nearby Eastvale complained about poor working conditions there.

In that case, Cal/OSHA fined NFI Industries, a subsidiary of New Jersey-based National Distribution Centers, and a staffing agency, Warestaff, about $57,000 last year. NFI operates the Eastvale warehouse, and the staffing agency supplied workers.